1. Field of the Invention
This invention concerns measuring characteristics, such as acidity and basicity, of insulating fluids, such as mineral oils and synthetic oils. Such measuring can monitor pipe corrosion and the degradation of internal combustion engine lubricating oil.
2. Description of the Prior Art
The acid or basic (pH) nature of fluids is important in many fields, particularly in the measurement or prevention of corrosion. For example, the transport of fluids through pipes of all sizes has generated much interest in monitoring the rate of corrosion in order to predict their life. Over the years many patents and standards have been created for the purpose of measuring corrosion rates. Much of this prior art is suitable for both electrically conducting and non-conducting fluids. Corrosion rates may be predicted from pH and other ion concentration measurements and this approach has been investigated. Specific applications, such as monitoring the degradation of internal combustion engine lubricating oil, have lead to very specific solutions.
The prior art for this invention is detailed in three sections. The first section covers the monitoring of metal corrosion in fluids. The second section concerns the monitoring of acidity/basicity (pH) of oils and other non-conductive fluids; and the third covers the measurement of oil degradation (or contamination) using capacitance measuring devices.
The definition of xe2x80x9ccorrosivexe2x80x9d is specific to both the fluid and the type of material used in each application and therefore is considered being simply the science of reaction of a solid with its environment. Corrosion in engineering usually is considered the reaction of a constructional material, such as metal, with its environment with a consequent deterioration in the properties of the material. The term xe2x80x9ccorrosivexe2x80x9d is used commonly to describe liquids or gases which are either acidic or alkaline in nature. Reference tables for metals, alloys and other engineering materials quote chemical compatibility data for mineral acids such as hydrochloric, sulfuric and nitric acid and in alkali metal bases such as potassium or sodium hydroxide.
Corrosion may be classified into the following types: Uniform, Localized, Selective Dissolution, Pitting and Interaction with a Mechanical Factor. Examples of Uniform corrosion include oxidation/tarnishing, active dissolution, anodic oxidation/passivity and chemical/electrochemical polishing. Dissolution is defined herein as the solubilization of a material. Erosion is the removal of material by some unspecified means and corrosion is a general term that encompasses dissolution, erosion and chemical reaction, such as oxidation and reduction. Localized corrosion often is due to heterogeneity in the material; and pitting occurs in passive metals in the presence of specific ions.
Many industries, such as petrochemical, chemical, pharmaceutical, and others have found it necessary to monitor the corrosion rates of fluid containers, piping, and other components used within corrosive environments. Component lifetime can be predicted and down time can be avoided by careful monitoring of the corrosion of critical components.
One of the first methods used for measuring corrosion was to detect the resistance changes within a piece of metal immersed in the corrosive liquid. The resistance of this sacrificial piece of metal changed with time and one embodiment of the method is described in U.S. Pat. No. 3,857,094 Caldecourt (December 1974). This patent describes an electrically resistive bridge element assembly comprising a thin metal strip folded into two arms and forming the bridge itself, one surface of one arm being immersed in the corrosive liquid and the other surface and arm forming the reference section. One advantage described for this design is that it provides temperature compensation.
A more comprehensive approach to monitoring corrosion is described in U.S. Pat. No. 3,936,737 Jefferies (February 1976), which describes a electrically resistive multi-element device that purports to eliminate the temperature dependence and provides for extended element life.
Electrical resistivity of metals usually is very low and therefore resistance changes in the sensor element that occur during corrosion are very small. This causes the sensitivity of such devices to be poor. Typically, detection of the slight amounts of metal lost per hour, which may be in the region of millionths of an inch per hour, gives rise to short term probe resistance changes of less than a micro-ohm. Data is extremely temperature dependent and the measurement of these small resistance changes yields signal-to-noise ratio problems, giving rise to practical limitations in detection limits.
Further improvements in electrical resistance corrosion probes are disclosed in U.S. Pat. No. 4,019,133 (April 1977); U.S. Pat. No. 4,217,544 (August 1980); and U.S. Pat. No. 4,326,164 (April 1982), all of which made progress in design, both mechanically and electrically, with the intention of elimination of the effects of temperature changes.
U.S. Pat. No. 4,338,563 (June 1982) discloses a secondary temperature compensation method that compensates for temperature differences between corrosion monitoring element and reference element, as well as fluid temperature compensation. It is known that measuring the resistance itself causes local probe heating; and the corrosion reaction itself can cause some chemically derived temperature fluctuations.
U.S. Pat. No. 4,587,479 (May 1986) discloses a multiple compensation method that further improves the usefulness of electrical resistance corrosion probes.
The creation and use of resistance probes have been prolific and such probes are available commercially and are in common use. They have been applied within many different industries and used for both aqueous and non-aqueous systems. One important issue with the resistance monitoring of metals to determine corrosion rate is how the resistivity of the metal changes with temperature. Fluids, such as ICE (Internal Combustion Engine) oils, may be at operating temperatures up to 120xc2x0 C. or higher. The resistivity of some metals, for example lead, changes significantly within a typical ICE engines operating temperature range. A system monitoring the resistance of a lead electrode for example would detect a much greater change in resistance within the temperature range, than change due to low levels of corrosion.
ASTM G 31-72 (Reapproved 1995) is a Standard Practice for Laboratory Immersion Corrosion Testing of Metals and describes in detail how to avoid the pitfalls while performing laboratory tests, and is a very useful source of reference material for such tests.
Many standards have been initiated and adopted for monitoring corrosion rates. ASTM D 1275-96a is a Standard Test Method for Corrosive Sulfur in Electrical Insulating Oils, that describes the observation of color and surface changes occurring in a thin copper sheet, when immersed in the oil under test. This method is qualitative only and is only able to classify samples as either corrosive, or non-corrosive.
ASTM G 102-89 (Reapproved 1994) is a Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. It provides guidance in conversion of electrochemical measurements to rates of uniform corrosion. It details Corrosion Current Density and Polarization Resistance topics and is a very useful reference in this field, as is ASTM G 3-89 (Reapproved 1994) the Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing.
U.S. Pat. No. 4,130,464 (December 1978) teaches us an electrochemical method of evaluating the corrosion rates of metals; and such methods have been standardized and are described in the subsequent Corrosion Standard Section.
ASTM G 96-90 (Reapproved 1995) is a Standard Guide for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods.) It details both the Electrical Resistance and the Polarization Resistance method that involves interaction with the electrochemical corrosion mechanism of metals in electrolytes in order to measure the instantaneous corrosion rate.
ASTM G 59-97 Standard Test Method for Conducting Polarization Resistance Measurements also is a useful reference for providing guidance in the measurements of Polarization Resistance, which can be related to the rate of corrosion of metals at or near their corrosion potential.
The measurement of the pH of oil is a field of considerable interest within the automotive and trucking industries. One of the most important uses is to detect the onset of corrosion, due to depletion of oil additives. An internal combustion engine (ICE) lubricating oil is manufactured by adding quantities of chemical additives to a base stock oil. The quantity and type of these chemical additives are dependent upon the particular engine application; for example spark ignition engines use different oil from diesel engines. It is relevant to list the chemicals additives according to function: viscosity modifiers, anti-wear additives, dispersants, detergents, antirust additives, antifoaming agents, pour point depressants, antioxidants and bearing corrosion inhibitors. The compounds particularly of interest used in diesel engine lubricants impart a base reserve to the oil and prevent corrosive wear by neutralizing the sulfuric other acids caused by combustion and oxidation products. Typical base reserve compounds include metal sulphonates, various dispersants, and corrosion inhibitors, such as zinc dithiophosphates.
Heavy-duty diesel engine oil is required to operate reliably over a wide range of temperatures and for a considerable period of time. The oil becomes contaminated with both soluble and insoluble products of combustion wear and atmospheric particles. Diesel fuels contain additives and contaminants that are transferred to the lubricating oil during the combustion process. An example of such a contaminant is sulfur. Sulfur is acidic, as are other combustion byproducts, such as nitrogen oxide compounds. The oil manufacturer adds a basic compound which neutralizes sulfur and other acidic compounds. Therefore, a certain quantity of reserve basicity is built into the oil. During use, this oil reserve basicity diminishes, until eventually it becomes fully depleted. At such time, unless further chemicals are added, the oil is at the end of its useful life and the acid nature of the oil will corrode engine components and cause excessive wear. It is most important to be able to detect the onset of this acidic condition.
The performance of the oil is critical to the life of a heavy-duty diesel engine and for many years attempts have been made to standardize these and other lubricating oils. ASTM D 5967-97 is a Standard Test Method for Evaluation of Diesel Engine Oils in T8 Diesel Engines that addresses in comprehensive detail the tests standard within the industry for oil performance.
On-Road and Off-Road fleets send oil samples taken at regular intervals to Oil Testing Laboratories, because monitoring such data is critical to the long term continued performance of such vehicles. The results available from such laboratory tests include Total Acid Number (TAN) and Total Base Number (TBN), which are measurements related to the pH of the oil and are indicative of the onset of component corrosion. Total Base Number (TBN) is defined as the quantity of perchloric acid expressed in terms of the equivalent number of milligrams of potassium hydroxide that are required to neutralize a given sample according the specific method used.
Current standards for measurement of TAN and TBN are limited to laboratory based equipment. ASTM D 664-95 a Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration and ASTM D 4739-96 a Standard Test Method for Base Number Determination by Potentiometric Titration are examples of current standards.
Attempts have been made to measure the TBN or TAN in situ on the engine itself, thus alleviating the need for dispatch of samples to the laboratory. U.S. Pat. No. 5,023,133, (June 1991) titled Acid Sensor is an example of the prior art for the measurement of pH or its equivalent. This patent discloses an electrically conductive polymer device, which senses changes of acidity in a non-aqueous medium and is particularly suitable for the determining the alkaline reserve in a motor vehicle lubricating oil. The organic polymer sensor is described as being mounted inside of an oil filter, and is replaced when the filter is replaced. The life of this sensor is not compatible with the modern extended drain intervals demanded from modern heavy-duty diesel engine vehicle operators. This sensor, like most pH sensing devices, needs to be renewed or regenerated at frequent intervals, as system poisoning takes place quite frequently.
U.S. Pat. No. 4,741,204 (May 1998), titled Measurement of the Depletion of the Basic Additives in Lubricating oil, describes a corrosion-linked method that monitors the electrical resistance of a sensor made of copper, lead, mixtures and alloys thereof. The resistance of the sensor indicates the corrosion rate, which is correlated to the depletion of the additives. This device is limited in sensitivity, because of the very small resistance changes encountered.
U.S. Pat. No. 5,146,169, (September 1992), titled Reference Electrode and a Pair of Electrodes for Detecting the Acidity or Basicity of Oil, discloses a reference electrode formed of lead, zinc, tin, indium, cadmium, magnesium or any alloy thereof, which is used with a responding electrode made of a conductive solid. The electrodes generate a potential difference that varies with the acidity or basicity of the oil or sample under test. This use of metal/metal oxide electrode to measure pH for aqueous systems has been known for many years.
A further class of detectors used for monitoring the deterioration of oils is based upon the measurement of the dielectric constant of the oil itself. Changes in the capacitance of the oil are detected using a pair of spaced sensor capacitor electrodes that use the oil to be measured as the dielectric medium. Various forms of this device have been developed, an example is disclosed in U.S. Pat. No. 4,646,070 (February 1997). Whereas this and other similar devices measure or indicate oil deterioration, they are not specific to monitoring one parameter, such as acidity or basicity since they respond to changes in many aspects of the oil, such as liquid and solid contaminants. This and subsequent enhancements, such as using magnetic fields, are designed to identify more specific aspects of the deterioration of the oil for example magnetic or carbon particle quantification. It is important to understand that the prior art capacitance measuring devices use the oil under test is used as a dielectric medium.
The problems encountered with pH devices, when installed in xe2x80x9cOn-Linexe2x80x9d or xe2x80x9cIn-Linexe2x80x9d situations are their lifetime. xe2x80x9cOn-Linexe2x80x9d is defined as being located in the operating machine, but away from the main flow, such as at the end of a sampling tube; whereas, xe2x80x9cIn-Linexe2x80x9d is defined as being located within the main flow.
It is known to those experienced in the art of measuring pH that electrodes used for this purpose become poisoned easily and, therefore, need to be regenerated quite frequently. Any device used in extended operation must be reliable and not give false indications. Poisoned electrodes do not give correct pH indications.
As the pH, or acidity/basicity of insulating oils is related to the corrosion of a metal electrode surface in this work, poisoning can be a relevant issue. U.S. Pat. No. 4,566,949, (January 1986), details a rapid method for cleaning an electrochemical detector and provides good background understanding of cleaning metal electrode surfaces by applying an electrical waveform that both oxidizes and reduces.
The present invention provides a method and apparatus for monitoring, for example, the corrosion rates of conducting materials, when immersed in electrically non-conducting or conducting fluids. The resulting data can be subjected to subsequent interpretation, using pre-determined correlation data, to yield the fluid acidity or alkalinity, presence of a specific chemical or contaminant. The invention is particularly suitable for use with insulating fluids, such as mineral and synthetic oils. Conducting materials are chosen to be modified, for example corroded or soluble by the test fluid, and form the electrode plates of a capacitive sensor, when these plates are coated or attached to a modern electronic dielectric medium. This dielectric medium is chosen such that it and the electrodes form a capacitor with an easily detectable capacitance value, for example 50 nF. At least one electrode of the capacitive sensor is located in the fluid stream, such that there results a plate dissolution modification, erosion, or corrosion modification to one or both of the plate electrodes. This plate modification is detected as a reduction, or apparent change in plate area, giving rise to an easily detectable capacitance change. The capacitance change detected is related to rate of erosion, dissolution or corrosion, i.e. modification and, therefore, the concentration of the contaminant and/or specific chemical of interest. It is not necessary to use two electrodes that are modified; nor is it necessary to electrically expose more than the one electrode for capacitive modification by the fluid under test.
The presence of a less insulating emulsified fluid within the electrically non-conducting fluid, for example glycol or water in oil, is detectable as an increase in the measured capacitance by all of the sensors described herein.
In the preferred embodiment of the invention, the electrode plate material is coated upon a piezoceramic material, such as Lead Zirconate Titanate (PZT), which has both a suitable dielectric constant and forms a piezo responsive element. The advantage of using PZT or other piezoceramic material in conjunction with a suitable electrode material is its ability to form an ultrasonic transducer element. Ultrasonic vibration is used to accelerate the dissolution, erosion or corrosion, i.e. capacitive modification of the electrode plates, thus giving a considerable ability to optimize the sensitivity. Summation of the ultrasonic energy provided to the PZT sensor plate is recorded and correlated to the rate of plate modification.
Further details and advantages of this method and sensor will become more apparent in the following description and the accompanying drawings.